The present invention relates to a method to establish a sensitometry curve for a photographic medium such as film. A sensitometry curve means a curve, a characteristic table, or a set of density values and exposure energies, which enable a medium exposure value to be linked to its optical density. The sensitometry curve is still called the Hurter-Driffield curve. Optical media, especially films, generally have a known sensitometry response. The film""s response is an important datum for adjusting a number of cameras or devices taking the film. Among these, for example one can mention still cameras, development equipment, and film digitizing systems. The exact adjustment of these devices, according to the film""s response, enables the restoration, at the output of a processing chain, of images reproducing, as faithfully as possible, the scenes taken.
The sensitometry response of a photographic medium is sensitive to parameters like the manufacturing processes, the conditions, and storage duration of the medium. It can also vary in time and its prior knowledge can turn out to be inaccurate at the time the medium is processed. This difficulty can be overcome by establishing for each photographic medium a specific sensitometry curve that allows for its aging. The aging is allowed for both before and after development.
The invention has applications for all types of photographic media and, especially, photographic papers and films. While not being reserved solely for the field of the professional image, the invention mainly aims to establish sensitometry curves for films used in motion picture cameras.
To establish the sensitometry curve of a photographic medium, a sensitometry control is formed on a reserved part of the medium. Sensitometry controls generally comprise one or more ranges that are exposed with various exposure energies. These energies are known and carefully calibrated. Sensitometry controls comprise, for example, 21 ranges, subject respectively to various energy exposures, but uniform for each range.
In a motion picture camera, a series of 21 consecutive views can be exposed, taken in a leader part of a film, with increasing calibrated energies.
The sensitometry curve can easily be established by measuring the optical density in each range of the sensitometry control and by associating to these measurements the values of the exposure energies. The establishment of the sensitometry curve can be limited to the simple collection of the measurements, associated with their exposure values, or possibly be represented in graph form. The representation is generally produced as a logarithmic scale.
The accuracy of the sensitometry curve depends on the quality of the density measurements and the accuracy of the exposure of the various ranges of the sensitometry controls. In so far as the equipment used to form the controls and their reading is perfectly calibrated, the establishment of the sensitometry curve is not especially difficult.
Devices for forming the sensitometry controls with perfectly known exposure energies are however costly. Moreover, when many different cameras are liable to be used to produce the shots, it is necessary to make uniform the sensitometry controls produced by the exposure equipment of the various cameras. Thus the cameras must be provided with calibrated standardized exposure means.
These difficulties are obstacles to establishing and automatically allowing for a film""s sensitometry response.
The goal of the invention is to propose a method for establishing the sensitometry curve of a medium that enables the difficulties mentioned above to be obviated.
One goal in particular is to propose such a method that does not require an accurately calibrated exposure means for forming sensitometry controls.
One goal is also to propose a method enabling a reliable sensitometry curve to be obtained despite having especially rudimentary equipment on board the camera.
One goal is finally to propose such a method that enables the area of the sensitometry control to be limited to a smaller area of the photographic medium.
To achieve these goals the object of the invention is more precisely a method for establishing the sensitometry curve for a photographic medium, the method comprising: the formation on the medium of at least one sensitometry control by exposing many ranges of the medium with various exposure energies, the exposure energy of each range being modulated according to a spatial modulation profile (P(x)) identical for all the ranges; the capture of optical density values of the sensitometry control in each range and in regions corresponding to various values of the modulation profile; the formation of sensitometry curve sections, each section being formed from density values captured in various ranges of the sensitometry controls, but in regions corresponding to the same value of the modulation profile of the exposure energies; and the energy offset of the curve sections to obtain partial section overlapping corresponding to neighboring exposure energies.
In the sense of the invention, the sensitometry curve is considered, independently from its graphic representation, as a means enabling the optical densities to be linked to the exposure energies of a medium. It may be summarized as a table or a simple collection of numerical values linking the optical density of the medium to the exposure energy received by the latter.
At the time of the exposure of the sensitometry control, the value of the exposure energy supplied in each range is not known with any great accuracy. The uncertainty about the exposure energies originates essentially from the uniformity defects of the exposure light sources liable to equip the cameras, and in the inaccuracy of their calibration. When the exposure means are rudimentary, the uncertainty about the exposure energies can be significant.
In a preferred implementation of the invention method, the exposure energies of the various ranges can follow a regular or not determined progression. In addition, the progression can take place with reference to a known or not energy value. While the regularity or exact knowledge of the progression of the energies is an advantage, it is not essential. This aspect will be re-examined in the description that follows. The progression of the exposure energies can be increasing or decreasing.
The lack of sure information as to the real value of the exposure energies received by the photographic medium is somewhat compensated for by the sure information according to which the modulation profiles of the various ranges are identical. In this way, by energy offsetting the curve sections, according to the invention, one can combine the information coming from the various regions of the exposure ranges, for the various modulation values of the exposure energy. This combination enables a continuous sensitometry curve to be obtained.
It should be noted that after the energy offset of the curve sections, and obtaining a continuous sensitometry curve, this curve can again be assigned with a global energy error. This global error results from the absence of an absolute energy reference for at least one of the sections. The global energy error of the sensitometry curve is not however prejudicial to its use. In fact it does not affect the essential characteristics of the curve, such as its slope and inflexions.
The formation of sections under step c) may be done in graph form. However, it preferably comprises the association with each density value, of an exposure energy value estimated according to the range of the sensitometry control in which the density value is captured, and in addition, the formation of density value sets, each set containing respectively the optical density values captured in the various ranges of the sensitometry control but in regions corresponding to the same value (P) of the modulation profile. Thus, step d) of the method can comprise simply the uniform offset of all the energy values of the same set of data. The value sets here correspond with the curve sections.
More precisely, and according to one special implementation option of the steps c) and d) of the method, this can comprise respectively: the formation of density matrices whose columns, respectively rows, correspond with increasing density values, respectively decreasing, of the same set of values; the intercorrelation of the columns, respectively rows, in relation to at least one column, respectively row, taken as reference; the search for an energy offset, for each column, respectively row, corresponding to a minimum of an intercorrelation function of the columns, respectively rows; and the application of the energy offset to the estimated exposure energy values of the set of values of the matrix column, respectively row.
The intercorrelation function is, for example, a sum function that is performed on the rows of the matrix and that acts on the absolute value of a difference between the matrix elements belonging to one column corresponding to a section of the curve to be offset, and the matrix elements belonging to a column corresponding to a section of the curve selected as reference. Other conventional intercorrelation functions can be selected and in particular quadratic intercorrelation functions. Offsetting the sections means in relation to the section taken as reference, or in relation to an arbitrarily fixed reference.
The method as described above can be applied to monochrome photographic medium, black and white type, or to color photographic medium. In the first case, a single exposure source of the medium is sufficient. Each captured density value is then associated with a single exposure energy value delivered by this source.
In the second case, i.e. for color media, it is possible to determine one sensitometry curve for many sensitive layers of the medium. For example, one sensitometry layer is determined for each of the basic colors: red, green and blue. A source with three color components then supplies the exposure energy. The medium""s optical density is associated with a linear combination of the exposure energies for each of the colors.
As an illustration, the density D (x, y) at a coordinate point (x, y) of a monochrome medium will have the following form:
D(x,y)=S(E*P(x, y)).
In this expression S denotes a function representative of the sensitometry response of the photographic medium. Knowledge of the function S is given by the sensitometry curve. The term E denotes the exposure energy supplied by the source and P(x, y) the value of the energy modulation profile at the point (x, y). The value of P(x, y) is, for example, a value between 0 and 1 when the means used to perform the modulation is an attenuator, such as a filter.
For a color medium subjected to three monochromatic sources supplying respectively energies Ered, Egreen and Eblue the following expressions are obtained in the same way:
Dred(x,y)=Sred(Crr*Ered*Pred(x, y)+Cgr*Egreen*Pgreen(x,y)+Cbr*Eblue*Pblue(x,y))
Dgreen(x,y)=Sgreen(Crg*Ered*Pred(x,y)+Cgg*Egreen*Pgreen(x,y)+Cbg*Eblue*Pblue(x,y))
Dblue(x,y)=Sblue(Crb*Ered*Pred(x,y)+Cgb*Egreen*Pgreen(x,y)+Cbb*Eblue*Pblue(x,y))
In the above expressions the same letters denote the same variables or functions as those previously mentioned and the indices xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d show that these values or functions are specific to these colors. The indices Crr, Cgr, Cbr, Crg, Cgg, Cbg, Cbg, Crb, Cbb are the coefficients of the linear combinations.
Among these indices, indices Crr, Cgg and Cbb are near 1 whereas the other indices are generally less than 1, because of the spectral selectivity of the sensitivity layers of the photographic medium.
As shown previously, the exposure energies of the various ranges are preferably selected to follow a regular progression, increasing or decreasing with known deviations. This may be obtained very simply by controlling, for example, the intensity of the electrical supply current of the exposure source or the duration of supplying the source for a given constant intensity. The adjustment of the exposure duration profits from the light integration capacities by the photographic medium. The regular progression of the exposure energies of the various ranges facilitates the relative positioning of the energy values to construct a sensitometry curve.
When the energies do not follow a regular progression, there is an additional uncertainty for the value of each energy. In this case, the method can be completed by one or more steps to correct the estimated exposure energy values. Such a step comprises, for example: the association with each density value, of an estimated exposure energy value according to the range of the sensitometry control in which the density value is captured and according to an estimated value of the modulation profile (P) in the region of the range in which the density value is captured; and the uniform offset of the energy values associated with at least one set of density values captured in the same range of the sensitometry control, so as to tend to a single sensitometry curve.
The value P of the modulation profile corresponding to each region, that is initially not known, can be directly calculated from the energy offset for which the curve section has been assigned. More precisely the energy offset taken on a logarithmic scale is simply equal to the value of the function P in the relevant region.
Other characteristics and advantages of the invention will appear in the following description, with reference to the figures in the appended drawings. This description is given purely as an illustration and is not limiting.